HomeMy WebLinkAbout601334013 Geotech Assessment
STRATUM GROUP
145,1 Grant Street, Bellingham, W A 98225
Phone (360) 714-9409
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August 22, 2006
George McLane
1301 S. 3rd Avenue
19C
Sequim, WA 98382
Re: Geology Hazard Assessment
Tax Parcel 601334013
Coyle, Jefferson County, Washington
Dear Mr. McLane:
Stratum Group is pleased to present the results of our geology assessment of the above
referenced property. The purpose of this geology evaluation was to 1) determine the suitability of
the property for the siting of a residence, 2) qualitatively evaluate the risk of slope failures, and 3)
provide general site development and maintenance recommendations for development of the
property adjacent to a potentially unstable slope. This evaluation included a visual inspection of
the property and vicinity, a visual inspection of the steep slope, review of available geologic
mapping in the area, and inspection of the shoreline at the base of the slope.
SCOPE OF SERVICES
The scope of our services included the following:
1) Conducted a site visit to visually inspect the subject property including the slope
conditions, shoreline conditions, and relevant conditions in the vicinity of the
property.
2) Observed surface soil conditions on the slope and on the uplands above the slope
by excavating shallow hand dug test pits.
3) Prepared this report summarizing our findings, including an evaluation of the
feasibility of building a residence on the subject property, a qualitative evaluation
of the shoreline bluff stability, recommendations for site development, and
recommendations for further investigation, if necessary.
GENERAL GEOLOGY
Northwestern Washington has been occupied by continental glaciers at least four times during
the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying
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August 22, 2006
Parcel 601334013, Coyle, Washington. ~l~fftR~U~~UUII~O&O
Geology Hazard Assessment
interglacial periods, the underlying bedrock was eroded and a relatively thick layer of glacial
related and interglacial fluvial sediments were deposited over the underlying bedrock in the
vicinity of the subject property.
The Geologic Map of Surficial Deposits in the Seattle 30' x 60' Ouadrangle, Washington (Yount,
Minard and Dembrof, 1993) and the Geologic Map of West-Central Jefferson County,
Washington (Birdseye, 1976) indicate that the site is underlain by recessional glacial outwash
deposits. However, based on my site observations, the site is underlain by glacial till. The till
consists of very compact silt and clay with gravel and sand that was deposited directly by glacial
Ice.
SPECIFIC SITE OBSERVATIONS
The site location map is provided on Figure 1. The upland portion of the subject property is a
relatively level area with some towards the northwest. An area cleared of trees is located on the
upland area that is the location where the proposed home site is located. The southwest portion of
the property slopes down towards the southwest to Fisherman Harbor, an inlet at the southern
end ofthe ToandosPeninsula.
The upper part of the slope down to the Harbor slopes down at an angle of approximately 30
degrees for a distance of approximately 40 feet and then becomes steeper with slopes 45 degrees
or greater. The shoreline bluff slope is tree covered with mature trees consisting of Douglas fir,
western red cedar and madrone. A thick understory of evergreen huckleberry and salal covers the
ground under the trees. The slope is planer to divergent on the subject property and the trees on
the slope are straight.
The slope is underlain by compact sandy to gravelly silt and clay that I interpret to be the glacial
till. No springs or seeps or indicators of potential unstable conditions were present anywhere on
the slope.
The shoreline along the base of the slope consists of a beach/tide flat area. Fisherman Harbor is
very protected and essentially consists of a drowned valley. Very little wave action takes place
within the harbor. Hence the rate of erosion at the base of the slope is very low.
CONCLUSIONS AND RECOMMENDATIONS
The upland portion of the property is a suitable building site. A residence can be built within the
area indicated on the Figure 2 Site Sketch Map. It is our opinion that a residence located within
the area indicated on Figure 2 will be at minimal risk of being impacted by landslides or
shoreline erosion.
Stratum Group
File:8,7,06
August 22, 2006
Parcel 601334013, Coyle, Washington
Geology Hazard Assessment
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The following recommendations should be followed in developing the site:
1. No structures should be built within 15 feet ofthe top of the 3D-degree slope on the
southwest side of the property. This distance will be adequate to prevent damage from
settlement along the top of the slope or any shallow soil creep or surface soil movement near
the top of the slope.
2. A deck may be constructed within the IS-foot setback area as long as the deck is structurally
separate from the residential structure.
3. Native vegetation on the slope should be maintained. Removal of potential hazard trees or
limbing and topping of trees for view purposes is acceptable as long as the debris is not left
on the slope. Thinning of trees is also acceptable as long as mature trees are left on 3D-foot
center spacing between trees.
4. Storm water should be infiltrated into the ground and not concentrated near the top of the
slope. Infiltration can be accomplished by a level infiltration/dispersion trench located on the
northwest, northeast or southeast sides of the property at least 25 feet from the top of the
steep slope. Alternatively simply letting the roof run off drip onto the ground without a gutter
system will sufficiently prevent flow of water over the slope.
We do not anticipate that the development of the subject property will cause any negative
impacts on the stability of other slopes or the shoreline bluff as long as our recommendations are
followed. The septic drain field will not pose a risk to slope stability as long as the drain field is
located at on the northeast portion ofthe property.
Please note that there are inherent risks associated with building on lots near or adjacent to steep
slopes. These are risks that the building owner should recognize and be willing to accept. If
conditions appear different than those described in this report, or other concerns arise, we request
that we be notified so we can review those areas and modify our recommendations as required.
Stratum Group appreciates the opportunity to be of service to you. Should you have any
questions regarding our reconnaissance please contact our office at (360) 714-9409.
Sincerely yours,
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Dan McShane, L.E.G., M.Sc.
Licensed Engineering Geologist
Stratum Group
File:S, 7 ,06
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